LIMITATIONS OF AVAILABLE
ATCM TECHNIQUES
At our institution, both angular- and zaxis–
modulation techniques have been
in use for over 18 months for routine
scanning of patients. While each technique
has strengths, there are some limitations
with each technique. With angular
modulation, the user has to prescribe
a specific tube current value, which introduces
subjectivity in the selection of an
effective tube current–time product. On
occasion in scanners with an angularmodulation
technique, selection of higher
effective tube current–time product values
for scanning of larger patients may require
an increase in the scanning time (especially
if the operator believes that selected
or allowed effective tube current–time
product will not result in optimum image
quality).
Because z-axis modulation is a recent
innovation, there is a noticeable lack of
scientific documentation in the medical
literature regarding appropriate noise indexes
for specific sizes or ages of patients
and specific clinical indications. In addition
to selecting manufacturer-recommended
noise indexes, however, users
can restrict the range of tube currents
that can be used for scanning in order to
cap the maximum dose to ensure a minimum
image quality. However, use of an
arbitrary range of tube currents introduces
subjective constraints to the technique.
A lower minimum tube current
may result in reduced patient exposure,
which occasionally results in noisier images
in small patients who are scanned at
a substantially reduced tube current.
Conversely, large patients occasionally
receive higher tube current with z-axis
modulation than they would receive if a
fixed-tube-current technique was used in
order to maintain the selected image
noise. While ATCM results in better image
quality in this setting, it can result in
greater radiation exposure than would a
fixed-tube-current protocol in large patients.